The influence of weathering on index properties and undrained shear strength for the Charmouth Mudstone Formation of the Lias Group at a site near Banbury, Oxfordshire, UK

The Lias outcrop extends continuously from Dorset to Yorkshire in England, with outlying areas in Somerset and Wales. It underlies the transport routes between a number of major UK cities. Understanding the material properties of the Lias Group is therefore important for infrastructure construction and maintenance across England and Wales.

This study examines the influence of weathering on the engineering properties of the Charmouth Mudstone Formation (Lias Group) in light of recent developments in ground-investigation practice including: (i) the use of modern visual weathering classifications for soils and rocks; and (ii) the availability of large ground-investigation datasets from the construction of the High Speed Two (HS2) railway.

The variability in the undrained shear strength data was consistent with the moisture content, liquidity index and plasticity index of the samples, but they were poorer indicators than shown in previous studies. The visually-assessed weathering class and the depth below ground level were found to be more useful indicators of the undrained shear strength of the clay and mudstone samples of the Charmouth Mudstone Formation.